Endocrine System & Hormone Vocabulary

Overview of Hormones & the Endocrine System

• Stereotype-buster: “hormonal” ≠ only sex hormones; >50 distinct chemical messengers circulate from birth to death.
• Hormones = blood-borne, long-acting signals that complement the nervous system’s fast electrical messages.
  • Nervous system: action potentials along neurons → millisecond onset, millisecond offset.
  • Endocrine system: hormones through bloodstream → seconds-to-hours onset, minutes-to-days duration.
• Prime mission: maintain overall homeostasis (temperature, blood chemistry, stress handling, growth, reproduction, immunity, sleep, metabolism, etc.).
• Glands are small, scattered “rogues” (brain → throat → abdomen → gonads). Any structure that secretes a hormone qualifies as a gland.
• Two functional “bosses” in the body:
  • Nervous system (electro-chemical wiring)
  • Endocrine system (chemical broadcasting)
• Many hormones form cascades: Hormone 1 → triggers Hormone 2 → triggers Hormone 3 … (“chemical relay race”).

Hormone Chemistry & Solubility

• Two major chemical families:
  • Amino-acid based (peptides, proteins, amines) → usually water-soluble.
  • Steroid / lipid-derived (e.g. cholesterol-based) → lipid-soluble.
• Solubility dictates receptor location:
  • Water-soluble → cannot cross lipid membrane → bind extracellular receptors.
  • Lipid-soluble → diffuse through membrane → bind intracellular / nuclear receptors.
• Binding alters target-cell activity (↑ or ↓ gene transcription, enzyme rates, membrane transport, etc.).

Major Glands, Locations & Signature Hormones

• Hypothalamus (diencephalon floor)
  • “Neuro-endocrine boss”; monitors blood, issues releasing/inhibiting factors (TRH, CRH, GnRH, GHRH, etc.).
• Pituitary (sits in sphenoid’s sella turcica, attached by infundibulum)
  • Anterior (adenohypophysis – glandular):
    • TSH, ACTH, FSH, LH, GH, PRL, MSH.
  • Posterior (neurohypophysis – neural): stores/secretes ADH & OT.
• Pineal (epithalamus, posterior to thalamus)
  • \text{Melatonin} → circadian rhythm / sleep–wake.
• Thyroid (anterior neck, over trachea; butterfly-shaped)
  • Follicular cells → T3 (triiodothyronine) & T4 (thyroxine): set metabolic rate.
  • Parafollicular (C) cells → \text{Calcitonin}: ↓ blood Ca^{2+}.
• Parathyroids (4 pea-sized glands on thyroid’s posterior)
  • PTH: ↑ blood Ca^{2+} by bone resorption, kidney reabsorption, vit D activation.
• Thymus (superior mediastinum; large in infants, atrophies post-puberty)
  • \text{Thymosin} & \text{Thymopoietin}: mature T-lymphocytes.
• Adrenal glands (on kidney poles)
  • Capsule → Cortex → Medulla.
  • Cortex zones (superficial → deep):
    • \text{Zona Glomerulosa} → mineralocorticoids (e.g., \text{Aldosterone}) – Na^+/K^+ & pH balance.
    • \text{Zona Fasciculata} → glucocorticoids (e.g., \text{Cortisol}) – stress, glucose, inflammation.
    • \text{Zona Reticularis} → gonadocorticoids (androgens/estrogens).
  • Medulla (neuro-endocrine) → \text{Epinephrine} & \text{Norepinephrine} (fight-or-flight catecholamines).
• Pancreas (retro-peritoneal; dual function)
  • Endocrine – Islets of Langerhans:
    • \alpha-cells → \text{Glucagon}: ↑ blood glucose.
    • \beta-cells → \text{Insulin}: ↓ blood glucose.
  • Exocrine – acinar cells → digestive enzymes & bicarbonate.
• Gonads
  • Ovaries → \text{Estrogen} & \text{Progesterone}: secondary female traits, uterine cycle, bone health.
  • Testes (interstitial/Leydig cells) → \text{Testosterone}: male traits, spermatogenesis, muscle/bone growth.

Classic Regulatory Examples & Cascades

Blood-Glucose Control (Pie Example)

• Eat strawberry-rhubarb pie à la mode → blood glucose spikes.
  • Pancreatic \beta-cells release \text{Insulin} → glucose stored as glycogen/fat → \downarrow blood glucose.
• Skip food for hours → blood glucose falls.
  • \alpha-cells release \text{Glucagon} → glycogen breakdown & hepatic gluconeogenesis → \uparrow blood glucose.
• Dysregulation → diabetes mellitus (hypo-insulin) or insulinoma (hyper-insulin).

Hypothalamic–Pituitary–Adrenal (HPA) Axis – Stress

1. Stress signal (e.g., waking in burning house). Hypothalamus secretes CRH.
2. Anterior pituitary releases ACTH.
3. Adrenal cortex secretes cortisol + mineralocorticoids →
   • ↑ BP, ↑ blood glucose, ↓ immunity, ↓ reproduction.
4. Negative feedback: rising cortisol → inhibits CRH & ACTH release.

• Endocrine limb is slower to rise & fall than sympathetic nerve firing → lingering “post-stress” feeling.

Hypothalamic–Pituitary–Thyroid (HPT) Axis – Metabolic Thermostat

• Cold blood reaches hypothalamus → TRH → anterior pituitary TSH → thyroid releases T3/T4.
• T3/T4 enter nuclei of nearly every cell → ↑ glucose catabolism → ↑ ATP → heat (calorigenic effect).
• Rising T3/T4 feedback inhibits TRH/TSH once warmth restored.

Key Disorders & Clinical Correlates

• Hyper- / Hypo-secretion extremes underpin most endocrine diseases.
• Graves’ Disease (Marie’s case study)
  • Autoantibodies mimic TSH → thyroid over-produces T3/T4 (hyperthyroidism).
  • Symptoms: weight loss, heat intolerance, anxiety, insomnia, palpitations, exophthalmos (bulging eyes—orbital inflammation).
  • Treatment: anti-thyroid drugs, radioactive iodine, surgery.
• Diabetes Mellitus
  • Type 1: autoimmune destruction of \beta-cells → no insulin.
  • Type 2: insulin resistance → compensatory hyperinsulinemia → eventual \beta failure.
• Hyperparathyroidism: excess PTH → bone demineralization, kidney stones (author’s own elevated PTH example → potential low Ca²⁺ intake).

Anatomical Landmarks & Histology Snapshots

• Sella turcica (“Turkish saddle”): sphenoid depression housing pituitary.
• Infundibulum: stalk linking hypothalamus & pituitary.
• Thyroid follicle: colloid-filled sphere lined by simple cuboidal follicular cells; parafollicular C-cells in between.
• Adrenal cortex layered mnemonic: “Salt → Sugar → Sex” (Glomerulosa, Fasciculata, Reticularis).
• Pancreatic islet resembles pale-staining “mini-island” amid darker acini.

Nervous–Endocrine Interplay & Integration

• Hypothalamus = anatomical & functional bridge (neurons + hormone secretion).
• Sympathetic nervous system premotor signals can drive adrenal medulla secretion.
• Many target organs express both neurotransmitter & hormone receptors (redundancy & fine-tuning).

Real-World, Ethical & Practical Notes

• OTC melatonin exploited as sleep aid; chronic misuse may distort circadian cues.
• Performance-enhancing misuse of anabolic steroids (synthetic androgens) disrupts natural gonadocorticoid axis.
• Public stereotype reduction: mood swings, acne, pregnancy tears ≠ “just hormones” — endocrine network is multidimensional & omnipresent.
• Maintaining endocrine health: balanced nutrition (iodine, Ca²⁺, vit D), stress management, regular screening (thyroid panel, glucose, bone density).

Learning Aids Referenced in Lecture

• Guided worksheets (“practice” points not graded but highly recommended).
• Flash-card sets embedded in module for quick recall of gland → hormone → function.
• “Operation” drag-and-drop game: label glands/hormones, reinforce anatomy.
• Mnemonics: “FLAT PG” (FSH, LH, ACTH, TSH, Prolactin, GH), “Go Find Rex” for adrenal cortex layers.

Numerical / Symbolic Quick Sheet (LaTeX Notation)

• T_4 = Thyroxine (4 iodine atoms)
• T_3 = Tri-iodothyronine (3 iodine atoms)
• Ca^{2+} homeostasis: Calcitonin (↓) vs $PTH (↑)
• Blood glucose feedback:
  • Eat → \beta-cell: \uparrow Insulin → \downarrow [Glucose]
  • Fast → \alpha-cell: \uparrow Glucagon → \uparrow [Glucose]
• Adrenal cortex priority mnemonic: \text{G} > \text{F} > \text{R} (external → internal)

Bottom-Line Takeaways

• “We are hormonal all the time.” Every cell and every second involves endocrine influence.
• Cascades & negative feedback form self-correcting loops essential for life.
• Disruption—whether autoimmune, tumor, nutritional, or iatrogenic—destabilizes homeostasis and underlies a wide spectrum of disease.
• Respect the hormones: they quietly keep you “not dead.”